The ability to measure accurately the dimensions of certain features on a substrate can be of critical importance in many applications. As used here the term substrate is not intended to be limited in its meaning and is to mean any kind of base, or component member, in or on which a feature or a surface relief element is positioned. For example, substrates can contain electronic elements or components which form an electronic device and the dimensions of such elements or components can, to a large extent, determine the electrical properties and performance of the electronic device. Accordingly, accurate knowledge concerning such dimensions becomes important both in the design and fabrication of the device.
Further, in the field of diffractive optics, for example, the dimensions and placement of surface relief features on a substrate determine the optical properties and performance of an optical element so that accurate information thereof becomes important in the design and fabrication of such optical elements.
Hence, techniques for the measurement of such dimensions can be used as a powerful quality control tool in designing electronic, or optical, or other devices, particularly if such measurements can be easily performed, both quickly and accurately.
Currently used optical methods for measuring such dimensions use light image intensity threshold detection techniques, such as disclosed in the article of D. Nyyssonen, "A Practical Method for Edge Detection and Focusing for Linewidth Measurements on Wafers," SPIE 538 Optical Microlithography IV, 172-178 (1985). In accordance therewith a great deal of effort has been required in developing suitable algorithms for determining the edges of the feature whose edge-to-edge dimension is being measured from the imaged light intensity distribution with respect to the feature under examination. The combination of intensity amplitude and phase differences in the light images at the edges of the feature, as well as the cross-interference which occurs when the edges of the feature are within the same diffractive zone, create apparent off-sets in the positions of such edges. A suitable algorithm then has to be devised to take into account and to correct for such aberrations. Such techniques may often make use of a large number of "look-alike" standard features and provide for the matching of the unknown feature being measured with the closest "look-alike" standard, using a light image intensity threshold condition. It is often difficult to select the correct standard without some prior knowledge of the detailed physical and optical characteristics of the feature being measured and, when the measurement is made, it is often difficult to specify the acceptable operational tolerances for the measurement. Moreover, if the illumination, or other, conditions change, the equipment usually has to be re-calibrated.
Such techniques often do not permit sufficiently accurate measurements to be made because such measurements depend on a knowledge of the optical properties of both the feature and the substrate material and such properties are often not sufficiently well known. For example, diffraction effects and the details of the resultant diffraction patterns which occur during the measurements make an accurate interpretation of the edges of the feature difficult to achieve. Moreover, such techniques are not always easy to perform, nor can results therefrom be obtained quickly.
It is desirable then to provide a measurement technique which is more easily used and which accurately and quickly measures such dimensions but which does not require information as to the details of the diffraction effects involved or of the optical properties of the materials involved. Moreover, it is desirable that such technique be usable in a generally universal sense for measuring a large variety of different features without the need to devise special algorithms for correcting for the diffractive effects which may differ considerably from feature to feature.